Transformer device and method for manufacturing a transformer device

ABSTRACT

A transformer device includes a glass substrate having a first side and a second side arranged opposite the first side. A first recess is formed at the first side of the glass substrate. A second recess is formed at the second side of the glass substrate. The first and second recesses are arranged opposite to each other. A first coil is arranged in the first recess and a second coil is arranged in the second recess.

FIELD OF THE INVENTION

The embodiments described herein relate to transformer devices, andparticularly to transformer devices integrated into a glass substrate,coreless transformers, transformers having a ferromagnetic core, andmethods for manufacturing the same.

BACKGROUND

Inductors and transformers are used for signal processing such asprocessing of gate signals for power devices. In many applications, inparticular high power applications, the primary and secondary coil ofthe transformer can be operated at different voltages. Such transformersneed a reliable electrical insulation between the primary and thesecondary coil to prevent electrical breakthrough and malfunction of thepower devices. For example, a transformer can be used to couple a lowvoltage control unit for controlling a high voltage device directly withthe high voltage device. Other options for coupling a low voltage devicewith a high voltage device are fibre optics or integrated circuits onSOI-technology.

Transformers are often integrated into the integrated circuit (IC) ofthe control unit by using the upper metallisation layer of the IC forforming the primary coil. The upper metallisation is covered by an imidelayer on which the secondary coil is formed. Such transformers needadditional layers and have a limited dielectric strength due to theimide insulation. Furthermore, a thick imide layer may warp thesemiconductor substrate of the IC. Moreover, imide or other syntheticmaterials used as insulation may degas in sputter tools used for formingthe secondary coil which causes additional cleaning of the sputtertools.

SUMMARY

Specific embodiments described herein pertain to, without being limitedthereto, a transformer device having at least a first coil on a firstside of a glass substrate and a second coil on a second side of theglass substrate wherein the glass substrate forms an electricalinsulation between the first coil and the second coil. Furtherembodiments described herein pertain to, without being limited thereto,coreless transformers using a glass substrate to insulate the primarycoil from the secondary coil. Other embodiments described herein pertainto, without being limited thereto, transformers having a ferromagneticcore and a glass substrate which insulates the primary coil from thesecondary coil. Further specific embodiments described herein pertain tomethods for manufacturing a transformer device.

According to one or more embodiments, a transformer device is provided.The transformer device includes a glass substrate having a first sideand a second side arranged opposite the first side. A first recess isformed in the glass substrate at the first side of the glass substrate.A second recess is formed in the glass substrate at the second side ofthe glass substrate. The first recess and the second recesses arearranged opposite to each other. A first coil is arranged in the firstrecess and a second coil is arranged in the second recess.

According to one or more embodiments, a transformer device is provided.The transformer device includes a glass substrate having a first portionwith a first thickness and a second portion with a second thickness lessthan the first thickness. The first portion laterally surrounds thesecond portion. The second portion has a first side and a second sidearranged opposite to the first side. A planar primary coil is arrangedon the first side of the second portion of the glass substrate. A planarsecondary coil is arranged on the second side of the second portion ofthe glass substrate opposite to the primary coil. The primary coil andthe secondary coil are electrically insulated from each other by thesecond portion of the glass substrate.

According to one or more embodiments, a transformer device is provided.The transformer device includes a glass substrate having a first sideand a second side arranged opposite to the first side. A first planarcoil includes a conductor arranged on the first side of the glasssubstrate. The conductor has an end portion arranged distal to the firstside of the glass substrate and sidewalls. A second planar coil includesa conductor arranged on the second side of the glass substrate. Thefirst and the second planar coils are arranged opposite to each other.The conductor of the second planar coil has an end portion arrangeddistal to the second side of the glass substrate and sidewalls. A firstferromagnetic cover is over the first planar coil on the first side ofthe glass substrate. The first ferromagnetic cover is electricallyinsulated from the first planar coil and covers the sidewalls and theend portion of the conductor of the first planar coil. A secondferromagnetic cover is over the second planar coil on the second side ofthe glass substrate. The second ferromagnetic cover is electricallyinsulated from the second planar coil and covers the sidewalls and theend portion of the conductor of the second planar coil. The glasssubstrate is arranged between the first cover and second ferromagneticcover.

According to one or more embodiments, a method for manufacturing atransformer device is provided. The method includes providing a glasssubstrate having a first side and a second side arranged opposite to thefirst side; forming a first recess in the glass substrate at the firstside of the glass substrate; forming a second recess in the glasssubstrate at the second side of the glass substrate opposite to thefirst recess; forming a first coil in the first recess; and forming asecond coil in the second recess.

According to one or more embodiments, a method for manufacturing atransformer device is provided. The method includes providing a glasssubstrate having a first side and a second side arranged opposite to thefirst side; forming a first planar coil having a conductor on the firstside of the glass substrate; forming a first insulating layer on thefirst planar coil; forming a first ferromagnetic cover over the firstplanar coil; forming a second planar coil having a conductor on thesecond side of the glass substrate; forming a second insulating layer onthe second planar coil; and forming a second ferromagnetic cover overthe second planar coil.

Those skilled in the art will recognize additional features andadvantages upon reading the following detailed description, and uponviewing the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are included to provide a furtherunderstanding of embodiments and are incorporated in and constitute apart of this specification. The drawings illustrate embodiments andtogether with the description serve to explain principles ofembodiments. Other embodiments and many of the intended advantages ofembodiments will be readily appreciated as they become better understoodby reference to the following detailed description. The elements of thedrawings are not necessarily to scale relative to each other. Likereference numerals designate corresponding similar parts.

FIGS. 1A to 1J illustrates processes of a method for manufacturing atransformer device according to one embodiment.

FIG. 2 illustrates a transformer device according to one embodiment.

FIG. 3 illustrates a transformer device according to one embodiment.

FIG. 4 illustrates the relation between penetration depth of eddycurrents and frequency of an electric field for different materials.

FIGS. 5A to 5C illustrate imprint processes used for manufacturing animprint mask according to several embodiments.

DETAILED DESCRIPTION

In the following Detailed Description, reference is made to theaccompanying drawings, which form a part hereof, and in which are shownby way of illustration specific embodiments in which the invention maybe practiced. In this regard, directional terminology, such as “top”,“bottom”, “front”, “back”, leading”, “trailing” etc., is used withreference to the orientation of the Figure(s) being described. Becausecomponents of embodiments can be positioned in a number of differentorientations, the directional terminology is used for purpose ofillustration and is in no way limiting. It is to be understood thatother embodiments may be utilised and structural or logical changes maybe made without departing from the scope of the present invention. Thefollowing detailed description, therefore, is not to be taken in alimiting sense, and the scope of the present invention is defined by theappended claims. The embodiments being described use specific language,which should not be construed as limiting the scope of the appendedclaims.

It is to be understood that features of the various exemplaryembodiments described herein may be combined with each other, unlessspecifically noted otherwise. For example, features illustrated ordescribed as part of one embodiment can be used in conjunction withfeatures of other embodiments to yield yet a further embodiment. It isintended that the present description includes such modifications andvariations.

The term “lateral” as used in this specification intends to describe anorientation parallel to the main surface of a glass substrate.

The term “vertical” as used in this specification intends to describe anorientation, which is arranged perpendicular to the main surface of theglass substrate.

In this specification, a second surface of a glass substrate isconsidered to be formed by the lower or back-side surface while a firstsurface is considered to be formed by the upper, front or main surfaceof the glass substrate. The terms “above” and “below” as used in thisspecification therefore describe a relative location of a structuralfeature to another structural feature with consideration of thisorientation shown in the Figures.

The terms “magnetically soft core”, “magnetic core”, “magnetisable corestructure” and “ferromagnetic cover” intend to describe structuresformed by a “magnetically soft” material which can be easily magnetizedand re-magnetized upon applying of an external magnetic field. Examplesof magnetically soft materials are unalloyed iron, nickel-iron alloysand cobalt-iron alloys. Such materials do not remain magnetised, or onlyweakly magnetised, when the field is removed unlike “magnetically hard”materials.

The term “cross-sectional view” intends to describe a verticalcross-sectional view through the glass substrate from the first side tothe second side and through the structures formed on both sides of theglass substrate.

When referring to semiconductor devices, at least two-terminal devicesare meant, an example is a diode. Semiconductor devices can also bethree-terminal devices such as a field-effect transistor (FET),insulated gate bipolar transistor (IGBT), junction field effecttransistors (JFET), and thyristors to name a few. The semiconductordevices can also include more than three terminals. According to anembodiment, semiconductor devices are power devices. Integrated circuitsinclude a plurality of integrated devices.

The transformer devices described herein, in the following referred toas transformers, are devices which are manufactured separately fromsemiconductor devices or integrated circuits so that manufacturingprocesses used for forming the transformers do not interfere withmanufacturing processes of the semiconductor devices or integratedcircuits and vice versa. This provides more freedom in tailoring theprocesses according to specific needs.

The transformer is formed on both sides of a glass substrate asdescribed below. The glass substrate forms an electrical insulationbetween the first coil and the second coil of the transformer. The firstand second coils can also be referred to as primary and secondary coils.The glass substrate provides an electrical insulation having adielectric strength which is significantly higher than that of imidematerials or other synthetic materials usually used for insulation.Therefore, the voltage difference between the first coil and the secondcoil can be up to 10000 V or higher depending on the glass material usedand the selected glass thickness between the first coil and the secondcoil. Such transformers provide a suitable alternative to other signaltransmission devices such as fibre optics or devices based onSOI-technology which are comparably expensive.

With reference to FIGS. 1A to 1J a first embodiment of a method formanufacturing a transformer device is described.

As illustrated in FIG. 1A, a glass substrate or glass wafer 10 having aninitial thickness d₁ between about 200 μm to about 1000 μm is provided.Typically, the initial thickness d₁ can be about 300 μm. The material ofthe glass substrate 10 can be selected to provide a desired dielectricstrength as described further below. According to one embodiment, theglass substrate 10 can be comprised of borosilicate glass such as BF33.

The glass substrate 10 has a first side or surface 11 and a second sideor surface 12 arranged opposite to the first side 11. An etching mask 15having at least one opening 15 a, typically a plurality of openings 15a, is formed on the first side 11 of the glass substrate 10. Eachopening 15 a defines the location and size of a recess to be formed inthe glass substrate 10. Etching mask 15 can be of any suitable materialfor example a photolithographically structurable resist.

As shown in FIG. 1B, first recesses 21 are etched at the first side 11of the glass substrate 10 using etching mask 15. Recesses 21 can bewet-chemically etched, for example in a heated HF-solution. In furtherembodiments, recesses 21 can be dry-chemically etched. The resultingstructure is illustrated in FIG. 1B.

The processes illustrated in FIGS. 1A and 1B are repeated on the secondside 12, i.e. a further etching mask is formed on the second side 12having openings for defining recesses to be etched, and then the secondside 12 is etched to form second recesses 22. The resulting structureafter removal of the etching masks is illustrated in FIG. 1C. First andsecond recesses 21, 22 are arranged such that a first recess 21 isarranged opposite to a second recess 22, i.e. a first and a secondrecess 21, 22 forming a pair of recesses at opposite sides of the glasssubstrates 10. Typically, first and second recesses 21, 22 havingsubstantially the same size although different size and depth are alsopossible.

The glass thickness d₂ between the first recess 21 and the second recess22 of a pair of recesses is adjusted to provide a dielectric strengthwhich is sufficient to prevent electrical breakthrough between the firstcoil and the second coil of the transformer when operating thetransformer. Glass thickness d₂ can be in a range from about 40 μm toabout 100 μm. In one embodiment, glass thickness d₂ is about 100 μm.

In a further process as illustrated in FIG. 1D, a first seed layer 27 isformed on the first side 11 of the glass substrate 10. First seed layer27 lines the first recesses 21 and can be, for example, deposited bysputtering. First seed layer 27 can be, for example, a Ti or an Aglayer. A second seed layer can be formed at the same time orsubsequently on the second side 12 of the glass substrate. It is alsopossible to form the second seed layer in a later step. FIG. 1J showsremaining portions 28 a of a second seed layer 28 in a final structure.

In a further process, a moldable material 24, typically a thermoplasticmaterial, is applied to the first side 11 of the glass substrate 10. Themoldable material 24 at least partially fills the first recesses 21.Typically, moldable material 24 completely fills the first recesses 21and covers the first side 11 of the glass substrate 10. Moldablematerial 24 can be, for example a polymer material such as an acrylresist. PMMA (Poly (methyl methacrylate)) is one example.

In a further process as illustrated in FIG. 1E, a template 26 having aprojecting pattern 26 a is provided. The pattern 26 a defines structuresto be imprinted into the moldable material 24. For example, pattern 26 ahas projecting structures defining the shape of one or more trenches tobe formed in the moldable material 24.

The template 26, which can also be referred to as a master form, ispressed into the moldable material 24 such that the projecting pattern26 a is inserted into the first recesses 21. With respect to FIGS. 1F to1I details of the following processes are illustrated using the sectionencircled in FIG. 1E by the dashed line.

The moldable material 24 is typically heated to become flowable. Theheating temperature typically exceeds the glass transition temperatureof the polymer forming the moldable material 24. As illustrated in FIG.1F, the template 26 is then pressed until the projecting pattern 26 areaches or nearly reaches the bottom of the first recesses 21.Typically, the projecting pattern 26 a does not reach the first seedlayer 27. The heating can also cause cross-linking of the polymermaterial. After cooling, the template 26 is removed.

In a further process, the moldable material 24 is suitably etched toremove the material left between the projecting pattern 26 a and thefirst seed layer 27 to expose the first seed layer 27 in the trenches 25a of the thus formed imprint mask 25. The above-described processes areone option for forming a mask by imprint lithography. Other options areillustrated in FIGS. 5A to 5C.

Briefly, FIG. 5A illustrates so-called “hot-embossing” as described inconnection with FIGS. 1D to 1F. A moldable polymer material 524 a isdeposited on a substrate 510 and heated above its glass transitiontemperature. Then a template 526 a formed of a comparably hard materialis pressed onto the material 524 a to transfer the pattern of thetemplate 526 a into the polymer material 524 a, which is subsequentlycooled. After cooling, the template 526 a is removed and the polymermaterial 524 a etched to complete imprint mask 525 a.

FIG. 5B illustrates so-called UV-imprint lithography using a template526 b which is transparent for UV radiation. Template 526 b is typicallya quartz glass substrate. UV-curable resist 524 b is deposited onto thesubstrate 510, for example drop-wise, and then template 526 b is pressedinto the flowable resist 524 b to impress the pattern of template 526 b.UV-radiation 580 is then used to cure resist 524 b which is etched afterremoval of template 526 b to obtain imprint mask 525 b.

FIG. 5C illustrates so-called micro-contact printing (μ-CP) using atemplate 526 c having a pattern which is dipped into a solution of aprint material to be transferred onto the surface of the substrate 510.Template 526 c is usually formed by a resilient material such assilicones, for example PDMS (polydimethylsiloxane). Template 526 c isthen gently pressed against the substrate to transfer the patternedprint material 524 c which finally forms an imprint mask 525 c on thesubstrate 510.

By using any one of the above or other suitable imprint processes, animprint mask 25 having at least one trench 25 a is formed in the firstrecess 21, with the trench 25 a extending to and exposing first seedlayer 27. The above processes can be summarised as imprint lithographyusing a template having a projecting pattern which is used to form animprint mask on the substrate.

Formation of mask layer 25 is, however, not restricted to imprintlithography. For example, photolithography can also be used for forminga mask having a high aspect ratio as described further below.

In a further process, as illustrated in FIG. 1G, a conductive materialis deposited into the trench or trenches 25 a. According to oneembodiment, the conductive material is copper or another suitable highlyconductive material which is electrolytically deposited, which can alsobe referred to as electroplating, using the first seed layer 27 asstarting layer. For example, a CuSO₄ solution can be used forelectrolytic copper deposition. Trenches 25 a are partially filled tokeep material of adjacent trenches 25 a separated from each other. Itshould be noted here that trenches 25 a illustrated in the Figures areparts of a single trench comprising one, two or more windings. FIGS. 1Fto 1J illustrates a trench having two planar spiral windings. In thecross-sectional view shown in the Figures, such a double winding appearas four trenches 25 a.

As illustrated in the Figures, trenches 25 a have a comparably largeaspect ratio, i.e. the trenches 25 a have a depth (or height) which islarger than the width of the trenches 25 a. In one embodiment, theaspect ration (height/width) is in a range from about 10 to about 50.Using trenches 25 a having such aspect ratio allows formation of aconductor 31 a having a similar or slightly less aspect ratio. Conductor31 a is formed by the conductive material deposited into the trenches 25a. High aspect ratio conductors have a large cross-sectional area andtherefore a reduced resistance which is beneficial for the quality (orquality factor; Q-factor) of the transformer.

The processes used to form the coils can also be described aspattern-plating process using a plating mask which is formed by mask(imprint mask) 25. Typically, the coils 31, 32 are planar coil, i.e. thewindings are formed substantially in the same level on the glasssubstrate.

In a further process, as illustrated in FIG. 1H, imprint mask 25 isremoved, for example by etching, to expose conductor 31 a which forms afirst coil 31 arranged in the first recess 21. Removal of imprint mask25 exposes sidewall portions and an end portion which is distal to theglass substrate 10. In FIG. 1H, this end portion is formed by upper endof the conductor 31 a. First seed layer 27 is then etched using theexposed conductor 31 a as mask. Portions 27 a of first seed layer 27covered by the conductor 31 a remain on the surface of the glasssubstrate 10.

The final conductor 31 a has a cross-section which is formed by thematerial of the remaining portions 27 a of the first seed layer 27 andthe deposited conductive material, for example copper. The total heighth is given by the thickness of the first seed layer 27 and the thicknessof the deposited conductive material as illustrated in FIG. 1H.Typically, first seed layer 27 is thin in comparison to theelectrolytically deposited conductive material so that the height h ofthe final conductor 31 a substantially corresponds to the height of theelectrolytically deposited conductive material.

In a further process, as illustrated in FIG. 1I, a passivation layer 47is deposited onto the first side 11 of the glass substrate 10.Passivation layer 47 completely covers first coil 31 formed by conductor31 a.

When see in a plan view onto the first side of the glass substrate 10,conductor 31 a of first coil 31 can be of any shape such as asingle-winding coil, a double-winding coil or a coil having more thantwo spiral windings. The windings can form, when seen in a plan view, aquadratic, elliptic, circular, rectangular or any other shape dependingon circumstances.

Imprint mask 25 also defines regions where pad structures are formed toprovide electrical contact to the first coil 31. FIG. 2 illustrates atransformer having first pad structures 41 a and 41 b which are inelectrical connection with ends of conductor 31 a of the first coil 31.These pad structures 41 a and 41 b are typically also formed within thefirst recess 21 and are exposed by etching the passivation layer 47.

Processes illustrated in FIGS. 1D to 1I are repeated on the second side12 to form a second coil 32 having a conductor 32 a which is formed byelectrolytically deposited conductive material and portions 28 a of thesecond seed layer 28. Finally, the glass substrate 10 is sawed orotherwise cut to from separate transformers. A resulting structure isillustrated in FIG. 1J with 71 denoting the sawing edge, 48 denoting apassivation material encapsulating the second coil 32, and 75 denotingan insulation material used for encapsulating the transformer.

A skilled person will appreciate that other processes for forming firstand second coils 31, 32 can also be used such as printing or pasting.When coils having a high aspect-ratio conductor are desired, imprintlithography or photolithography using a mask formed on the glasssubstrate 10 is typically used since these techniques allow an easyformation of high aspect masks.

The transformer as described in connection with the embodiment of FIGS.1A to 1J can have a symmetrical arrangement with respect to the size andlocation of the first and second recesses 21, 22 and the location of thefirst and second coils 31, 32 in the respective recesses 21, 22. Firstand second coils 31, 32 are arranged opposite to each other on differentsides of the glass substrate 10. The electrical insulation between thetwo coils 31, 32 is provided by the glass substrate 10 which has a highdielectric strength and can withstand high voltage differences. It istherefore possible to reduce the glass thickness d₂ between the firstand second recess 21, 22 to improve electromagnetic coupling between thefirst and second coils 31, 32. This in turns improves the quality of thetransformer.

Glass substrates have a comparably high dielectric strength incomparison to imide resists or other synthetic materials. Therefore, thetransformer as described herein can be designed to withstand highvoltage differences between the primary coil and the secondary coil(first and second coils) while otherwise providing sufficientelectromagnetic coupling between the coils.

The geometrical shape of the glass substrate also improves dielectricinsulation. The thin portion of the glass substrate 10 between the firstand second recesses 21, 22 forms a second portion having a secondthickness while thick regions of the glass substrate 10 outside to thefirst and second recesses 21, 22, i.e. un-etched regions of the glasssubstrate 10, form a first portion having a first or initial thicknessd₁ as described above. The second thickness d₂ is less than the firstthickness d₁. The ratio between d₁/d₂ can be in a range from about 2 toabout 25. In one embodiment, the ratio d₁/d₂ is about 3. The thicknessd₂ of the second portion is designed to withstand the high voltagedifference between the first and second coils 31, 32 while otherwisereducing the distance between the first and second coils 31, 32. Forexample, thickness d₂ of the second portion can be selected such thatthe second portion (thinned glass region between the first and secondrecesses 21, 22) provides a dielectric strength sufficient to withstanda voltage difference which is about 3-times of the needed blockingvoltage. For example, the initial or first thickness d₁ can be about 300μm, each of the first and second recesses 21, 22 can have a depth ofabout 100 μm so that the thickness d₂ is about 100 μm. The depth of thefirst and second recesses 21, 22 can be larger than the height of theconductors 31 a, 32 a formed therein.

The “increased” thickness of the glass substrate 10 lateral to the firstand second coils 31, 32 is for controlled reduction of the electricfield strength. This is illustrated in FIG. 1J which shows the course ofthe equipotential or isoelectric lines 35 between the first coil 31 andthe second coil 32. The isoelectric lines 35 are comparably closelyspaced to each other in the region between the first and second recesses21, 22 which indicates a high electric field strength. Lateral to thefirst and second coils 31, 32, the isolectric lines 35 can spread withinthe thick glass portion so that the distance between isolectric lines 35increases, which increase corresponds to a decrease of the electricfield strength. This “reduction” of the electric field strength occurswithin the glass substrate 10 before the electric field “reaches” otherinsulating material. For example, insulation material 75 experiences asignificantly weaker electrical field than the thin or second portion ofthe glass substrate 10 between the first and second recesses 21, 22. Thelarge thickness d₁ of the glass substrate 10 therefore provides an“insulation match” to allow a “transfer” of the voltage differencebetween the first and second coils 21, 22 by reduced electric fieldstrength. Insulation material 75 having a lower dielectric strength thanthe glass substrate 10 can thus form a reliable dielectric insulation.The initial or first thickness d₁ outside of the first and secondrecesses 21, 22 can therefore be selected to allow sufficient“spreading” of the isoelectric lines 35 for this transfer. The thickerglass regions therefore form a lateral insulation or transition regionaround the transformer. Typically, the thin glass portion (secondportion) is laterally completely surrounded by the thick glass portion(first portion).

A person skilled in the art will appreciate that that the course of theisoelectric lines 35 in FIG. 1J is only schematic and indicates only theprincipal course.

The first and second coils 31, 32 are arranged within the first andsecond recesses 21, 22 to keep their distance small. The “air gap”between the two coils is defined by the thickness d₂ of the glasssubstrate 10. FIG. 1J also schematically indicates the course of themagnetic field lines 36. As stated above, the distance between the twocoils are kept as small as possible while maintaining sufficientdielectric strength. Pads for electrical connecting the respective coils31, 32 can be arranged within the respective recesses 21, 22 or outsideof the recesses 21, 22 on the first portion of the glass substrate 10.

FIG. 1J also illustrates that the respective conductors 31 a, 32 a ofthe first and second coils 31, 32 have a cross-section with a comparablehigh aspect-ratio which allows formation of laterally small transformerswhile keeping the electrical resistance of the conductors 31 a, 32 asmall.

The transformers described herein are suitable for high voltageapplications, for example for coupling of gate control signals to highvoltage IGBTs which are used for PWM (pulse width modulation) of currentdelivered to high power loads such as railway engines and generators ofwind turbines. Furthermore, the transformers can also be used for mediumor low voltage applications to replace other coupling means between acontrolling unit and the driven device.

The transformers can be manufactured in a cost-efficient manner.Particularly the used imprint lithography contributes thereto. Thetemplate 26 used for imprinting the trench pattern can be used severaltimes.

FIG. 2 illustrates a coreless transformer according to an embodiment.Similar as the coreless transformer illustrated in FIG. 1J, the corelesstransformer of FIG. 2 includes a glass substrate 10 having two recesses21, 22 arranged on opposite sides of the glass substrate 10. A firstcoil 231 having four windings 231 a in this embodiment is arranged inthe first recess 21 while a second coil 232 having four windings 232 ais arranged in the second recess 22. A skilled person will appreciatethat the number of windings can be selected according to circumstancesand does not need to be the same for the first and second coils 231,232. First contact pads 41 a, 41 b are arranged in the first recess 21lateral to the first coil 231 while second contact pads 42 a, 42 b arearranged in the second recess 22 lateral to the second coil 232. Firstand second coils 231, 232 can be formed by same or similar processes asdescribed above. Second contact pads 42 a, 42 b are provided with aconductive glue 46 for connecting the second coil 232, which may formhere the primary coil (low voltage side) of the transformer, with an ICwhich drives the transformer. Alternatively, a solder material can beused instead of a conductive glue. First coil 231 forms the secondarycoil (high voltage side). The transformer can be mounted, for example,to an IC or another device by gluing, soldering or any other suitableprocess. The transformer can also be encapsulated by a passivationmaterial either alone or together with the IC.

The width A of the first and second recesses 21, 22 is selected toprovide sufficient space for integrating each of the coils 231, 232including their respective contact pads 41 a, 41 b, 42 a, 42 b. Contactpads 41 a, 41 b, 42 a, 42 b can also be formed outside of the recesses21, 22 on the thick glass portions of the substrate 10. First contactpads 41 a, 42 b are contacted by respective bond wires 45.

As described above, a plurality of transformers are formed together inthe glass substrate 10 and are finally separated from each other bysawing or other suitable cutting means to obtain individualtransformers.

While FIGS. 1A to 1J and 2 illustrate coreless transformers, FIG. 3illustrates a transformer having a ferromagnetic core according to anembodiment. Similar as described in connection with above embodiments, aglass substrate 310 having a first recess 321 and a second recess 322arranged opposite to the first recess 321 is provided. First and secondcoils 331 and 332 are formed in a respective one of the first and secondrecesses 321, 322 as described above. First and second seed layers forelectrolytical deposition of conductive material and their remainingportions after etching are not shown here for ease of illustration.

In a further process, a first insulating layer 351 is formed on thefirst side of the glass substrate 310. First insulating layer 351 iscomparably thin, for example 5 μm, and does not need to be designed towithstand the voltage difference between the first and the second coil331, 332. First insulating layer 351 merely provides an electricalinsulation between the first coil 331 and a subsequently formedferromagnetic cover. First insulating layer 351 is designed to withstandvoltages which occur due to self-inductance of the coils. These voltagesare significantly lower than the voltage between first and second coils331, 332. For example, first insulating layer 351 can by an oxide layersuch as a silicon oxide layer formed by CVD or other suitable depositionprocesses.

As indicated in FIG. 3, first insulating layer 351 conformally coversthe exposed conductors 331 a of the first coil 331 such that spacesbetween adjacent conductors or wirings 331 a are not completely filledby the first insulating layer 351. The thickness of the first insulatinglayer 351 is therefore significantly less than half of the distancebetween adjacent conductors 331 a. Hence, a clearance remains betweenadjacent conductors 331 a which will be filled with a ferromagnetic ormagnetically soft material. The remaining clearance between the adjacentwindings 331 a is selected such that formation of eddy currents in theferromagnetic material is reduced or avoided.

In a further process, a further seed layer 391 is deposited, for exampleby sputtering. A mask 385, for example a photo resist layer indicated bydashed-dotted lines, is formed to cover regions where no ferromagneticcover shall be formed.

Subsequently, ferromagnetic material such as iron is electrolyticallydeposited on exposed regions of the seed layer 391. Electrolyticallydeposited iron is comparably pure and has a high magnetic permittivity.A suitable plating bath can include iron chloride (e.g. 375 g/l) andcalcium chloride (e.g. 185 g/l). The pH-value of the plating bath isadjusted to be between 1 and 2, and the bath is heated to a temperaturebetween 90° C. and 110° C. Using a current density of about 4 to 20A/dm² an iron layer having low hardness and good ductility can beformed. Such a material is suitable for forming a first ferromagneticcover 361. A skilled person will appreciate that other plating solutionsand other ferromagnetic materials can be used as well. Theelectrolytically deposited iron can have a thickness sufficient tocompletely encapsulate the conductors 331 a and to form a closedferromagnetic cover 361. For example, iron can be deposited to athickness of about 100 μm which is sufficient for many applications.Other thicknesses are possible as well.

In a further process, mask 385 is removed and the further seed layer 391etched to remove the same from regions outside of the ferromagneticcover 361 using the ferromagnetic cover 361 as mask. It is also possibleto remove portions of the first insulating layer 351.

The same processes are carried out at the second side to form secondcoil 332 covered by a second ferromagnetic cover 362. The thus formedferromagnetic covers 361, 362 are subsequently annealed. Furtherprocesses include forming of passivation layers and suitable electricalconnections to the first and second coils 331, 332 which are not shownin FIG. 3.

First and second ferromagnetic covers 361, 362 form together aferromagnetic core which includes a gap formed by the thinned glasssubstrate 310 between the first and the second recesses 321, 322. Theglass substrate 310 separates the first ferromagnetic cover 361 from thesecond ferromagnetic cover 362. The gap d_(L), which substantiallycorresponds to d₂, may reduce the Q-factor of the transformer. Gapd_(L), which is formed by the thin portion of the glass substrate 310and the respective insulating layers 351, 352, can be, for example, in arange from about 40 μm to about 110 μm. This is, however, acceptable formany applications. On the other hand, conductors 331 a and 332 a of thefirst and second coils 331, 332 are completely surrounded by theferromagnetic material of the ferromagnetic covers 361 and 362 except onthe side facing the glass substrate 310. This configuration improves thetransformer quality and at least partially compensates for the gapformed by the glass substrate 310. The magnetic field lines 336 areindicated by dotted lines in FIG. 3 and are constrained substantiallywithin the first and second ferromagnetic covers 361, 362 while passingthrough the “gap” formed by the glass substrate 310.

The transformer having such a ferromagnetic core 361, 362 exhibits ahigh quality (Q-factor) and is suitable for signal transmissions in thekHz region up to the MHz region. The geometrical parameters of theferromagnetic core, i.e. of the first and second ferromagnetic covers361, 362, and the conductors 331 a and 332 a can be adapted in view ofthe signal frequency.

For example, the conductors 331 a and 332 a can have a high aspect-ratioand can be narrowly spaced so that the clearance between adjacentconductors remains small. In this case, only small “fingers” of therespective ferromagnetic covers 361, 362 are formed. These fingers formthe laminates of the ferromagnetic covers 361, 362 are preferably thinenough to avoid generation of eddy currents. The same applies to thewidth of the conductors. This will be described in connection with FIG.4 showing the penetration depth of the electric field into differentmaterials depending on the frequency.

FIG. 4 illustrates the penetration depth δ of an alternating electricfield for coal (1), Manganin® (2) which is a copper-manganese-nickelalloy, lead (3), tin (4), brass (5), aluminium

(6), copper (7) and silver (8). The penetration depth δ of analternating electric field into a conductive material can be calculatedusing the following relation:

$\delta = \sqrt{\frac{2}{\omega\kappa\mu}}$with ω denoting the angular frequency of the alternating electric field,κ denoting the electrical conductivity of the conductive material, and μthe permeability of the conductive material. The thickness of thelaminates or fingers of the ferromagnetic covers 361, 362 as well as ofthe conductors is preferably less than the penetration depth δ tosuppress generation of eddy current. Different penetration depths δapply for the conductors and the fingers due to the different materialsused. For example, when signals having a frequency of about 1 MHz areconsidered, the width w of the conductors is preferably less than 50 μm.Formation of such small conductors can be easily achieved by the abovedescribed processes. For example, the imprint masks 25 can be formedwith trenches 25 a having a width of 10 μm or less. Similar estimationscan be done for the ferromagnetic covers 361, 362. The width of theferromagnetic fingers or laminates should be in a similar range as thatof the conductors and is typically less than the width of theconductors.

Transformers suitable for transforming signals up to the MHz area cantherefore be formed. This is sufficient for many applications such asbridge circuits formed by power devices for transmitting gate signals.

For illustration purposes only, a non-limiting example is subsequentlydescribed. The conductors of the respective first and second coils havea cross sectional area with a width of about 10 μm and a height of about100 μm yielding an aspect ratio (h/w) of 10. A single-winding coilcovering an area of about 100 μm by 100 μm by forming a quadraticwinding, which has the above cross-sectional dimension, has aninductance of about 1 nH. The inductance of the respective coil can beincreased by increasing the number of the windings. Therefore, under theassumption that the total area available for the coil including its padsand its lateral insulation formed by the thick glass substrate is about1 mm² or less, coils having an inductance in the range up to few tens nHcan be formed.

Increasing the height of the conductors does not significantly changethe inductance but decreases the conductors resistance. The qualityfactor Q of a coil can be estimated by using the following relation:

$Q = {\frac{1}{R}\sqrt{\frac{L}{C}}}$with R being the resistance of the conductor, L the inductance and C thecapacitance formed by the conductor. For a copper coil having dimensionsas described above, a quality factor between few hundreds and up tothousand and more can be estimated. When using a ferromagnetic cover asdescribed above, the quality factor can be significantly increasedfurther which also improves the coupling between the two coils.

Herein described are embodiments of transformers having ahigh-dielectric strength insulation between the primary and thesecondary coil. Primary and secondary coil are separated and insulatedfrom each other by a glass substrate having a high dielectric strength,allowing the coils to be arranged close to each other while maintainingsufficient dielectric insulation. Each of the coils can be, for example,a double-winding formed by a conductor having an aspect ratio(height/width) significantly larger than 5, typically larger than 10 oreven 20. The lateral insulation of the coils is formed by a glasssubstrate or a portion of the glass substrate having a larger thicknessthan the glass substrate arranged directly between the coils to providea dielectric transition region to “pass” the high voltage differencebetween the coils to dielectric material having a lower dielectricstrength than the glass substrate. To this end, the glass substrate caninclude two recesses formed on opposite sides of the glass substrate,wherein each of the recesses accommodates a respective one of the twocoils.

Each of the two coils can be covered by a respective ferromagnetic coverso that the conductors are covered on sidewall and upper portions by theferromagnetic cover. The ferromagnetic covers are separated from eachother by the glass substrate. The ferromagnetic covers can also bearranged in the recesses. The magnetic flux passes from oneferromagnetic cover to the opposite ferromagnetic cover through theglass substrate.

The transformers have a comparably small size and can be manufactured ina cost-efficient manner on a glass substrate or wafer, with a pluralityof transformers formed on the glass wafer which is subsequently cut toseparate different transformers.

Terms such as “first”, “second”, and the like, are used to describevarious elements, regions, sections, etc. and not intended to belimiting. Like terms refer to like elements throughout the description.

As used herein, the terms “having”, “containing”, “including”,“comprising” and the like are open ended terms that indicate thepresence of stated elements or features, but do not preclude additionalelements or features. The articles “a”, “an” and “the” are intended toinclude the plural as well as the singular, unless the context clearlyindicates otherwise.

With the above range of variations and applications in mind, it shouldbe understood that the present invention is not limited by the foregoingdescription, nor is it limited by the accompanying drawings. Instead,the present invention is limited only by the following claims and theirlegal equivalents.

What is claimed is:
 1. A transformer device, comprising: a glasssubstrate comprising a first portion with a first thickness and a secondportion with a second thickness less than the first thickness, the firstportion laterally surrounding the second portion, the second portionhaving a first side and a second side arranged opposite the first side;a planar primary coil arranged on the first side of the second portionof the glass substrate; a planar secondary coil arranged on the secondside of the second portion of the glass substrate opposite to theprimary coil, the primary coil and the secondary coil being electricallyinsulated from each other by the second portion of the glass substrate.2. The transformer device according to claim 1, wherein the firstportion of the glass substrate laterally surrounds the primary andsecondary coils.
 3. The transformer device according to claim 1, furthercomprising: a first insulating layer on the primary coil and a firstferromagnetic cover over the primary coil, wherein the first insulatinglayer electrically insulates the primary coil from the firstferromagnetic cover; a second insulating layer on the secondary coil anda second ferromagnetic cover over the secondary coil, wherein the secondinsulating layer electrically insulates the secondary coil from thesecond ferromagnetic cover; wherein each of the first and secondinsulating layers has a dielectric strength less than a dielectricstrength of the second portion of the glass substrate.
 4. Thetransformer device according to claim 3, wherein each of the first andsecond ferromagnetic covers at least partially extends between adjacentwindings of the respective primary and secondary coils.